Thermoplastic resin microporous membranes are used widely as a material for separation, selective transmission and isolation of substances and the like. For example, the usage includes battery separators for lithium ion rechargeable batteries, nickel-metal hydride batteries, nickel-cadmium batteries, or polymer batteries, separators for electric double layer capacitors, various filters such as reverse osmosis filtration membrane, ultrafiltration membrane, microfiltration membrane and the like, moisture permeation waterproof clothes, medical materials and the like. Especially, polyethylene porous membranes are preferably used as separators for lithium ion rechargeable batteries. This is because the membrane exhibits ion permeability due to electrolytic solution impregnation and possesses not only electrical insulating properties, electrolytic solution resistance, and anti-oxidation properties, but also pore blocking characteristics (shut-down characteristics), which block the electrical current to prevent excessive temperature increase at the temperature range from about 120 to 150° C. in abnormal temperature increase in batteries. However, if the temperature continues to increase even after the pore blocking for some reason, the decrease in viscosity of the polyethylene that configures the membrane and the shrinkage of the membrane may lead to membrane puncture at a certain temperature.
Especially, separators for lithium-ion batteries greatly affect battery characteristics, battery productivity and battery safety, and require good mechanical properties, heat resistance, permeability, dimensional stability, shut-down characteristics, membrane melt-puncture characteristics (melt-down characteristics) and the like. Furthermore, they require improved adhesion with an electrode material for improvement in cycle characteristics of batteries and improved wettability toward electrolytic solution for productivity improvement. To improve these functions, lamination of various modified porous layers on a porous membrane has been studied previously. As modified porous layers, polyamideimide resin, polyimide resin, and polyamide resin, which have both good heat resistance and good wettability toward electrolytic solution, and/or fluorine-based resin, which exhibits good adhesion toward electrodes are preferably used. A modified porous layer described in the present invention refers to a layer that includes resin, which provides or improves at least one of the functions among heat resistance, adhesion with an electrode material, wettability toward electrolytic solution and the like.
Furthermore, it is necessary to increase the area not only of an electrode but also of a separator to fill a container in order to increase battery capacity Therefore the decrease in thickness of the separator is expected. However, since the thinner porous membrane may experience deformation in its planer directions, a modified porous layer laminated on a thin porous membrane in a battery separator may delaminate during processing, the slit process, or the battery assembly process and the safety may be compromised.
In addition, significant reduction in battery assembly process time is expected to facilitate cost reduction. Therefore the improvement in adhesion, which can withstand such a high-speed process with few problems such as delamination of modified porous layers, is required. However, when the resin included in the modified porous layer is sufficiently permeated into the polyolefin porous membrane, which is a substrate, in order to improve the contact, large increase in air permeation resistance has been problematic.